Male contraceptive gel effective in monkeys, but will it work in humans?

By Meera Senthilingam, CNN

Updated 1:42 PM ET, Tue February 7, 2017

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Photos:Birth control methods

A woman's choice – Women have many choices when it comes to avoiding pregnancy, but men have only two. According to the Centers for Disease Control and Prevention, 99% of sexually active women used at least one contraceptive method at some point between 2006 and 2010. Here's a look at a variety of birth control methods and how they each work.

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Photos:Birth control methods

A male condom is a thin covering worn on the penis during intercourse. It can prevent sexually transmitted diseases, and is about 82% effective at preventing pregnancy.

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Photos:Birth control methods

During a vasectomy, a surgeon cuts the vas deferens, the tubes that carry sperm from the testicles. It has a failure rate of about 0.15% and can be reversed, but the procedure is complicated.

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Photos:Birth control methods

IUD – An intrauterine device, or IUD, is a flexible T-shaped device that's inserted by a doctor into a woman's uterus. The devices block sperm and change the lining of the uterus, which may keep a fertilized egg from attaching. Pregnancy is prevented from three to 12 years, depending on the type.

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The pill – Approved in 1960 by the Food and Drug Administration, oral contraceptives involve taking a daily pill with a combination of estrogen and a progestin. The hormones prevent ovulation and thicken a woman's cervical mucus, blocking sperm from fertilizing an egg.

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Female condom – Condoms aren't just for men. The female condom fits inside the vagina with a ring at one end that covers the cervix. When used correctly all of the time, the National Institutes of Health says, it's 95% effective, with bonus protection from sexually transmitted infections.

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Diaphragm – The diaphragm also fits inside the vagina but covers only the cervix, where it blocks sperm from entering the uterus. It is made of silicone and can last up to two years. A spermicide must also be used for greatest effectiveness.

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Cervical cap – The cervical cap is similar to a diaphragm but smaller in size and made of rubber instead of silicone. It is useful for women who find it hard to keep a diaphragm in place.

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Implant – Implants are flexible matchstick-size devices that are surgically inserted into a woman's arm. They slowly release the hormone progestin into the body, preventing a woman's ovaries from releasing eggs. The protection can last several years.

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The patch – For women who don't want to take a pill or insert a device, the Ortho Evra birth control patch sticks to the body and releases pregnancy-preventing hormones through the skin. A woman must change her patch once a week for three weeks in a row. No patch is used in the fourth week, and then the cycle starts again.

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Vaginal ring – NuvaRing, approved for use in the United States in 2001, is a small, flexible ring inserted into the vagina. It releases estrogen and progestin, the same pregnancy-preventing hormones found in most variations of the birth-control pill.

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Vaginal sponge – Introduced in 1983, the Today Sponge was pulled from the market after fears of toxic shock but returned in 2005 following design changes. The sponge contains spermicide and can be inserted into the vagina before sex, like a diaphragm, to prevent pregnancy.

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The shot – Depo-Provera, an injection form of birth control, provides protection for three months with hormones that prevent ovulation and block sperm. It doesn't contain estrogen, as do some other forms of birth control. As a result, it is a popular option for women who can't take estrogen or who are breastfeeding.

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Morning-after pill – Emergency contraception, frequently referred to as the morning-after pill, can be taken to prevent pregnancy up to five days after unprotected sex. It can prevent the ovaries from releasing eggs and thickens a woman's cervical mucus. The morning-after pill can also thin the uterus lining, which could prevent a fertilized egg from attaching.

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Story highlights

Contraceptives for men involve one of two options: condoms or a vasectomy

An injection that block sperm from leaving the body has proved effective in monkeys

(CNN)When it comes to contraception, there are no long-lasting, easily reversible options for men. The only choices are condoms or a vasectomy.

But a new injectable gel that blocks sperm inside the body, preventing them from leaving while allowing seminal fluid to pass through, has proved effective in trials using monkeys. The researchers behind the product, known as Vasalgel, hope it could get trials in humans within a few years.

"What was important here was that this worked and was safe in animals similar to humans," said Elaine Lissner, executive director of the Parsemus Foundation, the medical research organization behind Vasalgel that is also leading the trials.

A new study, published Tuesday, involved 16 male monkeys receiving injections of Vasalgel who were released into social outdoor housing with females. Any adverse effects, as well as their ability to impregnate females, was compared with a historical sample of monkeys that had received vasectomies.

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"It was a simple efficacy study," Lissner said, meaning the researchers merely set out to see whether the gel was safe and worked to prevent pregnancy.

The social groups were monitored for at least one breeding season, and the team reported in its paper that none of the females in the group conceived during that time.

One monkey had the gel inserted incorrectly, and another developed a sperm granuloma, or lump of sperm, but the majority tolerated the gel and saw little inflammation, Lissner said, adding that rates of granuloma and inflammation are usually higher after a vasectomy.

Lissner's team is hoping to conduct clinical trials in humans next year and plans to stage further animal trials this year in preparation. But although the gel worked well in monkeys, it won't necessarily translate perfectly to humans.

"This is a bigger, longer task than we first anticipated," Lissner said.

Stopping sperm in their tracks

Administering Vasalgel inside a man, or a monkey, involves a polymer injected into the sperm-carrying tubes between the testicles and penis, known as the vas deferens, to block sperm as they are released.

The injection is given under anesthetic and mimics the effect of a vasectomy, but without the snip.

"It's a sticky gel that goes into (the tubes) and basically filters out sperm," Lissner said.

A similar polymer called RISUG is being trialled in India and has proved effective in men.

The new gel is injected in a location similar to that cut during a vasectomy and allows fluids through while blocking sperm. The backed-up sperm are then reabsorbed by the body.

A further benefit is that -- unlike with a vasectomy -- the gel is meant to disintegrate when a second solution is injected to break down the barrier and flush it out. This would make the process reversible.

Reversibility was shown in trials on rabbits, but it was not tested in the new trial on monkeys.

Recent trials on baboons also failed to reverse the effect.

"Successful reversibility will be the key to make this world-changing," Lissner said.

Invasive procedure

A further factor likely to impact numbers using this method is that Vasalgel still involves minor surgery to pull out the sperm tubes and inject the gel. What's more, when sperm are reabsorbed into the body, there is a risk of an immune reaction, potentially leaving men less fertile after reversal -- similar to vasectomy, though risks with the gel are lower, according to Lissner.

"Fifty percent of men after a vasectomy reversal remain infertile," saidDr. Ilpo Huhtaniemi, emeritus professor of reproductive endocrinology at Imperial College London, who was not involved in the new study. However, he points out that this doesn't prevent men from having children, as sperm can still be collected and used for in-vitro fertilization.

The latest evidence on monkeys has many hoping a new form of contraceptive may be on the horizon, but experts believe more evidence is first needed on its reversibility.

"In order for it to have a chance of replacing the traditional surgical method of vasectomy, the authors need to show that the procedure is reversible, and it's reassuring that apparently such studies are ongoing," said Allan Pacey, professor of andrology at the University of Sheffieldwho was also not involved in the new research. "The idea of trying to replace the traditional method of vasectomy by inserting a gel into the tube which carries sperm from the testicles to the penis at ejaculation is not a new one. However, we haven't seen much progress in developing the idea in recent years, so this study is a useful step in the right direction."

A male hormonal birth control injection was shown to prevent pregnancy in a 2016 study but was ended early due to side effects. Other approaches have involved strategies to control the production of sperm by targeting the hormone testosterone, but this had side effects of depression and weight gain.

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In 2015, Japanese scientists discovered a protein, calcineurin, found to be crucial in helping sperm swim and break through the membrane of a female egg in order to fertilize it. When the genes behind this protein were blocked during studies in mice, they became infertile.

But Vasalgel is further along than the Japanese research and has not shown severe side effects in animals. Lissner is confident that her team's gel will overcome the challenges. She sees value in providing birth control that does not interfere with hormones.

"Why manipulate the whole body when there's a small tube that all the sperm swim through?" she asked.

Huhtaniemi agrees that Vasalgel could be valuable in certain populations but feels that in terms of birth control for the masses, a less-invasive option is needed.

"We need a pill. That's the easiest to administer," he said. "It is feasible. We just need to find the right target."